An ophthalmic lens blank generally has a first face with a predetermined curvature and a second face and an outer edge. The second face is opposite the first face on which a desired surface contour is generated by a machining process. The overall process is generally referred to as “lens surfacing” and the overall object is to yield a finished spectacle lens so that the first and second face curvatures cooperate to yield desired optical properties. The first and/or second faces of the lens are usually coated to provide the finished spectacle lens with an enhanced ability to resist scratching (by means of a “hard coating”), with a low residual reflection and a desired color (by means of an “antireflection coating”), and/or with certain surface properties such as hydrophobic, oleophobic and dust repelling properties (by means of a “top coating”). Usually also a further machining process takes place (the so-called “edging”), the aim of which is to finish-machine the edge of the spectacle lens in such a way that the spectacle lens may be inserted into a spectacle frame. In all these process steps the spectacle lens (blank) must somehow be held in the machining machine(s) and coating apparatus respectively.
In more detail, hitherto the following main process steps are usually carried out in prescription workshops: Firstly, a suitable right and/or left ophthalmic lens blank is removed from a semifinished product inventory. The term “semifinished” is used to mean that the spectacle lens blanks, which are usually round or oval in plan view and have not yet been edged, have already been molded, machined or in another way contoured (surfaced) on one of their two optically active faces only. The spectacle lens blanks are then prepared for the blocking operation, namely by applying a suitable protective film or a suitable protective lacquer to protect the optically active face which has already been machined or contoured, i.e. the first face or blocking face.
The so-called “blocking” of the ophthalmic lens blanks then takes place. During this operation, the spectacle lens blank is joined to a suitable block piece, for example a lens block according to German standard DIN 58766, EP 1 593 458 A2 or document EP 2 266 753 A1. To this end, the block piece is firstly brought into a predefined position with respect to the protected first face of the spectacle lens blank, and then in this position the space between block piece and spectacle lens blank is filled with a molten material (normally a metal alloy or wax) or an adhesive composition that is curable, e.g., by UV or visible light, as described in the earlier European patent application EP 2 011 604 A1 by the same applicant for instance. Once this material has solidified or cured, the block piece forms a holder or support for machining the second face of the spectacle lens blank. The block piece is grasped by a chuck or other suitable coupling means during lens generation to provide in particular secure mounting to the profiling machine while avoiding damage to the lens.
Lens surfacing is carried out using profiling machines which typically have a cutter of some type that is moved across the second face of the ophthalmic lens blank to give the second face its macrogeometry according to the prescription. The spectacle lens blank may be stationary or rotating during the cutting operation, depending on the particular profiling generator (which is) being used. Typical machining processes for surfacing spectacle lenses include single point diamond turning (as the presently preferred fine cutting process for plastic materials and described in, e.g., document EP 1 719 585 A2 by the same applicant), diamond tool milling (as the presently preferred rough cutting process for plastic materials and described in, e.g., document EP 0 758 571 A1 by the same applicant), and grinding processes, applied depending on the lens material.
Usually fine machining of the ophthalmic lenses then takes place, in which the pre-machined second face of the respective spectacle lens blank is given the desired microgeometry, as described, e.g., in documents EP 1 473 116 A1 and EP 1 698 432 A2 by the same applicant. Depending on inter alia the material of the spectacle lenses, the fine machining process is divided into a fine grinding operation and a subsequent polishing operation, or includes only a polishing operation if a polishable second face has already been produced during the pre-machining stage.
Only after the polishing operation, cleaning steps are carried out. Then the coating process(es) take(s) place that, depending on among other things the material of the lens blank, may include spin (or dip) coating of the spectacle lens blank so as to provide at least the second face of the lens blank with a hard coating or the like, as described, e.g., in the earlier U.S. patent application Ser. No. 11/502,306, wherein the spectacle lens blank is held in the spin coating apparatus by means of a lens holder that has a suction cup for instance.
After the coating step(s) the ophthalmic lens blank usually is edged so that the spectacle lens can be inserted into a spectacle frame. Finally, after edging the spectacle lens is cleaned again and ready for inspection and insertion into/mounting to the spectacle frame.
Document WO 2009/135689 teaches a device for blocking workpieces, particularly spectacle lenses, for the processing and/or the coating thereof. The device comprises a support for supporting the workpiece, an exposure device for exposing light to the blocking material for curing, from below the transparent block piece by an ultraviolet light source.
A UV light source for the curing stage is described in document WO 03/096 387 A2. A UV light source is associated with a lens block support for directing light through the lens block, which is in a material capable of transmitting at least the UV light. UV light is generated by the UV light source and transmitted through the lens block at a given wavelength, intensity and for a duration sufficient to cause the adhesive to cure, thereby bonding the lens blank to the block piece.
In this process, the lens blank, the adhesive and the block piece are cured together, which means that assembly formed by the lens blank, the adhesive and the block piece “freezes” together as a monolithic assembly. The polymerisation during curing induces shrinkage and thermal expansion from the chemical reaction. The increase in the temperature causes internal stresses in final assembly.
Shrinkage in turn may bend/distort or even shift with respect to the block piece the blocked lens blank so that the curve which is cut into the second surface of the lens may become distorted when the processed lens is deblocked from the block piece and resumes its natural shape. Different adhesive compositions have been developed in order to reduce the shrinkage and thermal expansion resulting from the exothermic reaction.
However, there remains room for improving the curing process, in particular for reducing the shrinkage resulting from the polymerisation.